[PATCH] Make docbook2man optional

module-init-tools build currently fails if docbook2man is not
available on the build system. However, since docbook2man is only used
to build the documentation, it is not strictly necessary, and there
are cases (embedded Linux build systems) where it's really useful to
not have to build things such as docbook2man.

This patch has been present in Buildroot for quite some time
now :

 http://git.buildroot.net/buildroot/tree/package/module-init-tools/module-init-tools-3.11-add-manpages-config-option.patch

OpenEmbedded has a different hack to disable the build of
module-init-tools manpages:

 http://git.openembedded.org/cgit.cgi/openembedded/tree/recipes/module-init-tools/files/no_man_rebuild

PTXdist has yet another different hack to also disable the build of
module-init-tools manpages:

 http://git.pengutronix.de/?p=ptxdist.git;a=blob;f=rules/module-init-tools.make;h=0af89157a0bec221bf44e44a8e66f9be5c23190f;hb=HEAD#l41

The fix consists in adding an automake conditional variable
HAVE_DOCBOOKTOMAN, which is defined when docbook-to-man has been
found. This variable is used in the Makefile.am to enable or disable
the construction of the manpages.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@...>
---
 Makefile.am  |    7 ++++++-
 configure.ac |    7 +++----